1. Field of the Invention
The invention relates to a porous-wall compact diffuser for dispersing the supersonic gas flow from the optical cavity of a chemical laser.
2. Description of the Prior Art
In chemical lasers, the reactants creating the optical emissions are mixed in a supersonic combustion process within the optical cavity. Heat release due to chemical reaction causes a reduction in the flow Mach number and an increase in static pressure within the combustion zone (Rayleigh process) for a constant area flow channel. The radial outflow nozzle designs currently in use prevent the pressure increase for the most part, but the Mach number decrease is primarily due to temperature rise and, therefore, still occurs in practice. However, the conditions at the exit of the optical cavity are still those of a hot, supersonic flow at low static pressures. Supersonic/subsonic diffusers are used to convert the remaining kinetic energy in the flow to increased static pressure in the exhaust flow. Because of the low cavity pressures and heat-release, the stagnation pressure at the diffuser entrance is less than atmospheric and must be either pumped by ejectors or large mechanical pumps or by direct absorption into chemically-active media.
Conventional diffusers are designed on the basis of oblique shock recovery limited by boundary layer flow separation. Because of the low Reynolds number flow common to chemical lasers, diffuser performance will be dominated by flow separation of the laminar boundary layers in the supersonic diffuser section. The weak oblique shock waves have relatively low pressure increases and thus avoid flow separation observed in strong adverse pressure gradients. For a conventional diffuser design consisting of a slight contraction, followed by a constant area section, and a diverging subsonic diffuser section, optimum performance is defined by conditions such that a weak oblique shock train terminates in the constant area section as a weak normal shock wave (sometimes referred to as a Mach disc). Design guidelines for diffuser constant-area section length generally are on the order of 10 entrance heights for a Mach 2 entrance flow.
Some conventional diffuser designs incorporate horizontal and/or vertical vanes to subdivide the main flow channel into several isolated flow compartments. The length-to-height ratio (L/H) requirement is then equivalent to a length-to-hydraulic diameter ratio (L/D.sub.H) generally used for flow channels with near unity height-to-width aspect ratios, where D.sub.H is four times the cross-sectional area divided by the linear dimensions around the flow channel. Test results have indicated that there is a practical limit to the number of vanes that may be added to a design, and the L/D.sub.H value remains at 10 to 12 for optimum pressure recovery. Furthermore, for vanes without an entering boundary layer the number of vanes increase exponentially as length decreases. For instance, four vertical vanes would be required to reduce diffuser length by a factor of 2. An additional length reduction by another factor of 2 would require 16 compartments. Additionally, the increase in wetted surface area due to the added vanes increases frictional losses which reduces overall pressure recovery. A physical limit on wall thickness is also approached due to minimum second throat area requirements for starting and running conditions. It is believed that the vaned diffuser designs would have overall length reduction potential limited to about L/H=6.